Remove setup_requires declaration from setup.py

[sbesson]

The pytest-runner is already declared in requirements-test.txt. This change should fix the installation of omero-mapr on old Python/pip (e.g. CentOS) following the PyPI TLS v1.0/1.1 deprecation

See also https://github.com/openmicroscopy/omero-marshal/pull/50

To test this PR,

• check Travis remains green
• test

$ docker run --rm -it centos:centos7
[root@985cea0418f9 /]# yum install -y python-virtualenv && virtualenv /tmp/venv
[root@985cea0418f9 /]# /tmp/venv/bin/pip install omero-mapr # should fail
[root@985cea0418f9 /]# /tmp/venv/bin/pip install https://github.com/sbesson/omero-mapr/archive/pypi_tls_1.0.zip # should work

[joshmoore]

:+1: for green travis. Local testing in docker as described passes.

Note: pip install -U setuptools pip also fixes the issue.

Propose to release ASAP as 0.2.2.